1. Field of the Invention
This invention relates to an electrical circuit controller for protecting a power circuit and a load powered by the power circuit, and more particularly to a contactor having an external tungsten series current limiter for providing low cost current limiting protection in the power circuit.
2. Background of Information
Electromagnetic contactors are electrically operated switches used for controlling motors and other types of electrical loads. Contactors include a set of movable electrical contacts which are brought into contact with a set of fixed electrical contacts to close the contactor and connect a power line to the load. The set of movable contacts are separated from the set of fixed contacts to open the contactor.
Contactors also include a magnetic circuit having a fixed magnet and a movable armature with an air gap therebetween when the contactor is opened. An electromagnetic coil is controllable upon command to interact with a source of voltage for electromagnetically accelerating the armature towards the fixed magnet, thus reducing the air gap. Disposed on the armature is the set of movable contacts. The complementary set of fixed contacts are fixedly disposed within the contactor case and engage the movable contacts as the magnetic circuit is energized and the armature is moved. The load and voltage source therefor are interconnected with the fixed contacts and become interconnected with each other as the movable contacts make with the fixed contacts.
Contact erosion in contactors primarily occurs during a contact breaking cycle. During such cycle, the separable contacts (i.e., the fixed contacts and the movable contacts) part and the current flowing therethrough forms an arc. Continued arcing eventually interferes with the ability of the separable contacts to conduct electricity. The surface of the separable contacts may become eroded, pitted or may have carbon build-up.
A motor-starting contactor with a thermal overload protection relay system is called a motor starter. The purpose of the overload relay is to sense heat produced by line current and "trip" or stop the motor if the retained heat exceeds an acceptable level. State of the art overload protection relay systems include current sensors which output a voltage proportional to the current. After an analog-to-digital conversion of the voltage, a microprocessor squares and integrates the converted digital value to achieve a true measure of motor heating. This approach provides an accurate degree of motor protection. However, the overload relay cannot effectively interrupt a short circuit current. Without additional circuit protection, the short circuit current may damage the separable contacts of the contactor.
Circuit breakers are generally used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload fault or a relatively high level short circuit condition. An overload fault condition is normally about 125-600 percent of the nominal current rating of the circuit breaker. A high level short circuit condition can be 1000 percent or more of the nominal current rating of the circuit breaker. For example, in a bolted three phase line-ground fault, the short circuit current may exceed 100 KA at 480 VAC.
In conventional motor starter installations, a three-phase power source powers a motor through a circuit breaker and a separate motor starter having an overload protection relay. The circuit breaker generally provides both overcurrent protection and power circuit disconnection functions. Under short circuit fault conditions, the circuit breaker acts first to protect the power circuit. This is because the motor starter trip characteristics are generally designed for interrupting, after an I.sup.2 t time delay period, persistent overload currents associated with motor overloads. In the event of a motor starter failure, involving welded contacts, the circuit breaker may also be used to disconnect the power source from the motor.
Various proposals have been advanced for limiting short circuit fault currents in conventional motor starter installations. International Application Number PCT/SE91/00076 discloses a circuit breaker comprising a trip circuit, a trip coil, a set of contacts responsive to the trip coil, and a positive temperature coefficient conductive polymer thermistor in series with the set of contacts. The thermistor limits the magnitude of the short-circuit current. The coil of the trip circuit rapidly opens the contacts within approximately 5 ms after a fault current exceeds five to ten times rated current.
It has also been proposed to incorporate a nichrome current limiting resistive element in series with the line conductor of a circuit breaker in order to limit the maximum short circuit current.
These proposals require a circuit breaker for rapidly interrupting the short circuit fault current. Furthermore, they are subject to an internal temperature rise because of the resistive (I.sup.2 R) heating of the current limiter. This temperature rise is especially critical whenever a load, such as a motor having an initially high inrush current, is repetitively cycled by periodically opening and closing the contactor.
There is a need, therefore, for a circuit controller for a load, such as a motor, which does not require a circuit breaker and which is not subject to the internal temperature rise of a current limiter.
There is a more particular need for such a controller which protects a power circuit from damage under short circuit conditions.
There is an even more particular need for such a controller which is not damaged or seriously degraded under short circuit conditions.
There is another even more particular need for such a controller which interrupts short circuit currents.
There is yet another even more particular need for such a controller which limits the arcing of the separable contacts.